Where information is transmitted to a selected recipient, it is becoming increasingly important nowadays to adequately protect this information in order to prevent it from being wrongfully used by third parties. This concerns, e.g., information to be displayed on screens and to be seen by selected viewers only, such as, e.g., engineering drawings exclusively intended to be displayed by selected monitors in a company's internal network; the input of so-called PIN codes at payment terminals and automated teller machines; or the transmission of transaction numbers for banking transactions (TAN); especially, it concerns the transmission of personal identification numbers (PIN), which are required, e.g., to use bank or credit cards, or to enable SIM (Subscriber Identity Module) cards for cellular phones.
In banking transactions, for example, so-called mobile TANs—in some countries called OTPs (Online Transaction PINs or One-Time-Passwords) are frequently transmitted by SMS; the user then needs to enter them in a certain box on a display connected to, or integrated with, a terminal. It is possible, however, for a so-called man in the middle to interfere with the transmission of the SMS or its display on the recipient's screen, since SMSs are usually not encrypted. Where PINs are supplied, these are usually mailed to the recipient, who will find the PIN, e.g., under a scratch-off layer or a strip-off label. The PIN itself, though, is not encrypted, which makes it easier to misuse by a third person who intercepts the mail.
For this reason it is desirable to encrypt and transmit such information in such a way that the probability of it being decrypted and read by persons other than the intended recipient is kept as slim as possible.